1. Field of the Invention
The present invention relates to a process for recovering valuable metals from a waste catalyst or waste catalysts carrying predominantly alumina.
2. Prior Art
Catalysts comprising an active metal such as molybdenum, nickel, and cobalt on a catalyst carrier made of alumina or alumina containing a small amount of silica are generally used in the industrial field of organic chemistry. Those types of catalysts are used particularly as hydrodesulfurization catalysts in petroleum refining. However, the catalytic activity of these catalysts during their application to the hydrodesulfurization of petroleum is gradually impaired by heavy metals such as vanadium and nickel that are delivered from the petroleum into the catalysts. Accordingly, these catalysts must be disposed of after their use for a predetermined duration of time. The so-called waste catalysts are produced in this manner.
However, the disordered disposal of these used catalysts is problematic in various aspects. The recovery of precious metals from such waste catalysts has been found very important, not only from the viewpoint of preventing environmental pollution but also from the viewpoint of conserving and reusing resources. Accordingly, a variety of processes and methods have been proposed heretofore to seek solution of the aforementioned problem, and some of them have already been put into practice.
Known methods for recovering valuable metals from waste catalysts proposed to date can be classified into two types; one aims to recover only vanadium and molybdenum from the waste catalysts, and the other aims to recover not only vanadium and molybdenum, but also nickel and cobalt.
The methods belonging to the former type include those disclosed in U.S. Pat. No. 4,087,510 and JP-A-47-31892 (the term "JP-A-" as used herein signifies Japanese Patent laid-open application for public inspection). For example, the process disclosed in U.S. Pat. No. 4,087,510 comprises adding caustic soda or sodium carbonate to the waste catalyst to roast it so as to convert vanadium and molybdenum in the waste catalyst to water-soluble sodium salts, leaching the thus formed water-soluble sodium salts with water and separating and recovering vanadium and molybdenum from the leachate. The process disclosed in JP-A-47-31892 comprises subjecting a waste catalyst to an oxidizing roasting, then further roasting the thus roasted waste catalyst with addition of caustic soda or sodium carbonate, leaching the further roasted waste catalyst with water and separating and recovering vanadium and molybdenum from the leachate.
The aforementioned two processes comprise roasting the waste catalyst at a high temperature. This treatment allows nickel and cobalt to form a complex oxide with the alumina in the carrier, or allows alumina itself to undergo structural transformation from .tau.-Al.sub.2 O.sub.3 to .alpha.-Al.sub.2 O.sub.3 (corundum). A corundum-type structured alumina is extremely stable against acids and alkalis; thus, after the structural transformation, it happens that vanadium and molybdenum are selectively leached into the leachate from the thus roasted waste catalysts. Accordingly, the process above can be said ideal in selectively recovering vanadium and molybdenum, however, it is useless from the viewpoint of recovering all the valuable metals from the waste catalysts.
In view of the recent demands of conserving natural resources, there have been many proposals for recovering valuable metals such as nickel and cobalt in addition to vanadium and molybdenum from the waste catalyst, the proposals of which being made by JP-A-47-21387, JP-A-54-107801 or JP-A-51-73998, each disclosing a process for recovering valuable metals from the waste catalyst. That is, JP-A-47-21387 discloses a process comprising subjecting the waste catalyst to the oxidizing roasting to remove organic matter, sulfur or like therefrom and then leaching with use of ammonia water the valuable metals such as vanadium, molybdenum, nickel and cobalt from the waste catalyst in normal pressures or under pressure. JP-A-54-107801 discloses a process which comprises subjecting the waste catalyst to the oxidizing roasting at a temperature of from 300.degree. to 1000.degree. C., adding chlorine gas to the thus roasted waste catalyst to chlorinate the valuable metals such as vanadium, molybdenum, nickel and cobalt in the waste catalyst and leaching and recovering the chlorinated valuable metals. JP-A-51-73998 discloses a process which comprises roasting the waste catalyst in a vapor atmosphere to remove the organic matters from the waste catalyst and leaching with use of a highly concentrated acid the valuable metals such as vanadium, molybdenum, nickel and cobalt from the waste catalyst.
However, any of these prior art processes as above mentioned provides an insufficient leaching ratio with respect to nickel or cobalt and also an insufficient dissolution of alumina content so that it may result in obtaining an aluminum solution containing vanadium, molybdenum, nickel and cobalt as a leachate. In the recovery of vanadium, molybdenum, nickel and cobalt from such an alumina solution as above mentioned, the aluminum content in the solution will interfere with the extracting or recovering process and the insufficient content and the insufficient leaching ratio of the metals will also prevent an efficient recovery.
Therefore, it can be said that there has been no proposal of a process for efficiently recovering the valuable metals such as vanadium, molybdenum, nickel and cobalt from the waste catalyst on an industrial scale.